What were the "some transactions" that the credit union "let" go through?
If these were outstanding checks, closing the account doesn't change your obligations to the credit union. Nevada's bad check law makes a crime of closing an account in this way under most circumstances.
If you wrote checks after the account closed, you have an even larger criminal problem.
On the other hand, if these were "stale" checks, you may have something to work with.
As it stands, though, there isn't enough here to answer your question.
I would close your browser and run to a criminal defense attorney's office for a paid consultation at this point.
I changed your question's area to criminal defense, because that is your primary problem here. You need to take stock of every check you might have written that was not covered by the funds in the account. If you bounce a check for nonsufficient funds, or the account was closed in the State of Nevada, you can be charged with a felony.
If you think there are any outstanding checks that could bounce, you need legal help right away. Your debt to the credit union is the lesser of your concerns. You can always open another account later. Figure out the criminal end of this first.